1. Field of the Invention
This invention relates to a thermally developable light-sensitive material, and more particularly, to a thermally developable light-sensitive material which shows reduced thermal fog (undesirable fog caused in unexposed portions upon image-wise exposing and developing by heat) due to the presence of a palladium-containing compound.
2. Description of the Prior Art
Photographic processes using silver halide have been widely employed since such processes provide excellent photographic properties such as sensitivity and gradation as compared with electrophotography, diazo-type photographic process and other photographic processes. However, silver halide light-sensitive materials used for silver halide photographic processes must be developed with a developer after image-wise exposure, and then subjected to several bath processings for stopping, fixing, washing or stabilizing so as to prevent the developed image from becoming discolored or faded under normal light, and also to prevent undeveloped white portions (the background) from being blackened. Such processings require much time and are laborious, and, in addition, there are many problems such as the danger to humans handling the chemicals involved, contamination of processing rooms or contamination of the bodies or clothing of operators, and environmental pollution due to discharging the processing solutions into streams. Therefore, it has been desired to provide highly sensitive photographic materials containing silver halide which can be dry processed, i.e., without bath processings, which can provide stable images after processing and which also have a background that does not discolor under normal light conditions.
For this purpose, various efforts have been made. For example, it is described in German Pat. Nos. 1,123,203 and 1,174,157 that the incorporation of a 3-pyrazolidone developing agent into a silver halide emulsion enables development by heat, and in this case, the reaction is accelerated by the presence of a substance capable of supplying water on heating, as described in German Pat. No. 1,175,075. Furthermore, German Pat. No. 1,003,578 discloses that a silver halide fixing agent can also be present. However, these techniques cannot render silver halides completely stable to light which remain in a light-sensitive material after dry processing.
The three patents first mentioned above do not describe fixing in a dry processing, and with the invention disclosed in the latter patent, undesirable reaction occurs upon storage since the developing agent (reducing agent) and the fixing agent are present together, and therefore, its practical use is low.
At present, the most successful light-sensitive materials capable of forming a photographic image by dry processing are thermally developable light-sensitive materials which employ a composition containing, as essential components, a silver salt of an organic acid, a small amount of silver halide and a reducing agent, as described in U.S. Pat. Nos. 3,152,904 and 3,457,075. Although in such light-sensitive systems silver halide remains in the light-sensitive material after development and is not stabilized to light, they exhibit the same effect as in the case of stabilizing the silver halide, as silver halide is used in a small amount and the majority of the system is a white or light-colored stable organic silver salts which is not blackened by light. Therefore, even if a small amount of silver halide is discolored by light, the light-sensitive material appears white or light-colored as a whole, and such slight discoloration is not visually bothersome.
Such light-sensitive materials are stable at ambient temperatures, but when they are heated to 80.degree. C. or more, preferably to 100.degree. C. or more, after being image-wise exposed, the oxidizing agent for the organic silver salt and the reducing agent in the light-sensitive layer undergo a redox reaction due to the catalytic effect of the exposed silver halide near the agents to yield silver, by which exposed portions in the light-sensitive layer are rapidly blackened to contrast with unexposed portions (the background), thus forming an image.
However, in the thermally developable light-sensitive materials which have so far been proposed, e.g., in a combination comprising a silver salt of an aliphatic acid such as silver behenate, a reducing agent and a catalytic amount of silver halide, heating results in considerable blackening in unexposed portions (which is referred to as thermal fox hereinafter), which provides no contrast with the blackening (image density) in exposed portions produced by heating, so that the formed image becomes illegible. Therefore, it is an important subject to reduce thermal fog.
Moreover, when such light-sensitive materials are stored for a long period of time prior to use, particularly at high temperature (from 30.degree. to 50.degree. C.) and high humidity (relative humidity of 50% or more), thermal fog is also produced, and only illegible images can be obtained.
It is known that mercury compounds improve the aforementioned defects, as described in Japanese Pat. No. 11,113/72. However, mercury compounds are poisonous and, therefore, can cause serious environmental pollution problems, e.g., upon development by heat, mercury is evaporated off, or on using such a light-sensitive paper as regenerated paper, mercury escapes into streams.